Reference Guide for the Model RT210/RT211 ISDN Routers
B-4 TCP/IP Routing
An IP address alone cannot be used to deliver data from one device to another on a LAN. In order
for data to be sent from one point in the network to a destination elsewhere, we must convert the IP
address.
This IP address must be a unique identifier that identifies the destination device attached to the
Ethernet LAN. Each device on an Ethernet network has a unique Ethernet MAC address, which is
a 48 bit number assigned to each device usually by the manufacturer. It is possible for some
devices to have more than one MAC address if they have multiple connections to Ethernet
networks. The technique that converts the IP address to a MAC address so that they match up is
known as address resolution.
Unless the router can resolve the network IP address to the Ethernet MAC address, it cannot send
the data to the correct device on the LAN. IP uses the Address Resolution Protocol (ARP) to do
this.
If a device needs to send data to another station on the network and it does not have its destination
MAC address recorded, ARP is used. An ARP request is sent out onto the network, which all
stations receive and read. The destination IP Address for the desired station is included as part of
the message so that only the station with this IP address responds to the ARP request and all other
nodes discard it.
The node whose IP address is included in the ARP request responds with its own MAC address
directly to the sender. This provides the transmitting station with the destination MAC address
needed for it to send the data. The IP address data and MAC address data for each node is held in a
Host Cache or ARP table, so that the next time data needs to be sent, the address can be obtained
from the address information in the ARP table.
NGRRT210.BK Page 4 Friday, October 17, 1997 2:37 PM